Early vehicle climate control systems utilized resistor cards to provide discrete blower speeds for manual adjustment. These resistor cards introduced different resistances in line with the blower motor to set motor current. To cool the resistor card, the part was typically placed in the air stream of the blower system.
Innovation led to the use of “linear” devices based on circuits using a power bipolar junction transistor (BJT) in combination with a relay to achieve high blower speed. These devices were more adjustable than the resistor cards, but the requirement to dissipate the required power in linear mode mandated the use of large, heavy, and costly heat sinks. These devices typically received a signal from a control head and adjusted their outputs without regard to changing battery voltages or motor stall.
Other variable blower control designs have implemented the use of field effect transistor (FET) devices in lieu of the power BJT and relay. This is possible due to the low drain-source resistances available in the FET devices. However, the power requirements in the mid-blower speeds continue to require a large heat sink to dissipate the power. Variations to combat this shortcoming have included variable blower controllers with a pulse width modulated (PWM) output. Such controllers allow for a much smaller heat-sink. However, the switching of the controller has introduced a new problem of noise for the blower controller.